Peelable, protective films and tapes have a number of uses in industry. These uses include protecting finished surfaces from damage during manufacture, shipping, and handling as well as masking certain areas of a surface from exposure during spraying operations including painting. The coatings are generally cohesive films which have moderate adhesion to the substrate and which can be easily removed from the surface simply by lifting one portion from the surface and peeling the film away.
In masking applications, the films are generally relatively clear to allow the mask preparer to discern patterns projected onto and through the film. In addition, the films are relatively easily cut with sharp knives. This is to allow the mask maker to apply the film to a surface, project a pattern through a transparent or translucent substrate, and to cut through the film mask to selectively expose areas of the substrate to permit the painting of exposed areas of the substrate.
Masks are commonly used for protecting selected areas of a substrate to be treated with a surface treatment, such as spray painting, sandblasting, etc. Such masks are used for covering a part of the surface to be treated so that the covered surface may be part of the surface of the final product, or the covered surface may be later treated with a different surface treatment process. For example, a surface of an automobile may be painted with two different colors. Part of the surface to be painted with one color may first be masked while the rest of the surface is painted with the other color. Afterwards, the first area may then be unmasked and painted while the second area is masked.
Many varieties of masking tapes are commercially available. Some of these masking tapes contain tapes that are made of paper, while others contain tapes that are made of polymeric materials. Examples of commercially available masking tapes are, SCOTCH 233.TM. (3M Company, St. Paul, Minn.), ARMAC TG6.TM., American Tape.TM..
Masking tapes have been used for a long time. Efforts have been made to find improvements. Usually improvements can be made on the adhesive element or the tape element.
Early aqueous films are represented by Oppenheim, U.S. Pat. No. 2,680,724, which discloses particular temporary protective coatings based upon a plasticized polyvinyl butyryl resin. However, the aqueous composition requires processing at elevated temperatures, well in excess of 100.degree. C. This material has apparently been marketed for a number of years under the trademark SPRAYLAT. However, once the material has been used and stripped from a surface, the material does not appear to be recoverable, and the processor must dispose of a great deal of waste. It is well known that current manufacturing conditions are demanding a reduction in such process waste, and therefore, a recoverable masking or protective film must be found.
Another peelable or strippable film is disclosed in Martenson, U.S. Pat. No, 4,792,464. This film is a temporary, readily strippable, protective film for metal surfaces which incorporates corrosion inhibitors and is impervious to gases such as oxygen and water vapor. The film is applied as an aqueous composition comprising (1) a terpolymer of polyvinyl butyryl, polyvinyl alcohol, and polyvinyl acetate; (2) a hydroxyalkyl cellulose ester; (3) a corrosion inhibitor dispersed in a plasticizer for the polyvinyl butyryl; and (4) morpholine as a leveling agent. This composition is apparently designed simply as temporary, protective coating for metals, and the reference apparently does not contemplate use of the film as a painting mask. In any event, the reference fails to recognize the waste problem of requiring the recovery and reuse of the film.
A third type of removable, protective film composition is disclosed in Wagner, U.S. Pat. No. 5,010,131. This film is disclosed for use in paint spray booths to protect the booth and equipment therein from paint overspray. The film-forming aqueous solution comprises a film-former, a filler, an alkaline source, and a blowing agent. Film-formers disclosed include vinyl acetate copolymer emulsions, sugar, soap, certain organic salts, and polyvinyl alcohol. The film is sprayed on a surface and allowed to dry. After the film has become contaminated with paint overspray, it is removed by action of a pressurized spray of hot water (at least 190.degree. F.). The action of the hot water activates the blowing agent which helps to release the film from substrates. Thus, the film does not appear to be peelable. Further, the removal process results in the presence of potentially hazardous particulate waste material in process water. This creates significant waste removal problems for the practitioner. In addition, there is no disclosure of recovery or reuse of the protective film material.
Winslow, U.S. Pat. No. 4,374,883 describes a normally tacky and pressure sensitive adhesive tape that is resistant to aging. The adhesive of the pressure sensitive adhesive tape contains a sheet backing bearing a composite adhesive stratum having a layer of polyurethane and a layer of soft, normally tacky pressure sensitive adhesive bonded to the polyurethane layer. The pressure sensitive tape is described as having superior adhesive properties. Polymeric films and paper are used as backing for the adhesive.
Another example of efforts to find better adhesives for a pressure sensitive adhesive tape suitable for a masking tape is Darvell et al., U.S. Pat. No. 4,855,170. Darvell et al. discuss a pressure sensitive tape having a pressure sensitive adhesive containing and completely surrounding resilient non-tacky hollow thermoplastic polymeric microspheres. The presence of the polymeric microsphere in the adhesive is described as responsible for enhancing desirable mechanical properties such as compression recovery, high peel strength, improved static sheer and high tensile strength. Examples of tape materials for coating with the adhesive are cellulose acetate and polyester. There is no emphasis on the properties of the tape materials.
Sometimes, special materials are incorporated into the tape part of the masking tape to impart special properties. Wenhold et al., U.S. Pat. No. 4,917,926 describe a pressure sensitive adhesive tape for masking selective areas of an article containing a polymeric film coated with a layer of adhesive, wherein both the film and the adhesive are resistant to high temperatures and soluble in a fluorocarbon solvent. This masking tape is particularly applicable in soldering operations involving printed circuit boards. Wenhold et al. also point out that water soluble tapes based on polyvinyl alcohol are also possible. Such solvent possible tapes are adapted for ready removal when the surface treatment has been accomplished.
Since masking tapes are sometimes applied on surfaces that are not completely flat, it is necessary for a masking tape to be able to conform to the contour on the object on which the tape is to be applied. Kusunoki et al., U.S. Pat. No. 5,178,913 describes a printing mask containing an aluminum foil and a film of a resin provided on one surface of the foil and having a high power of adhesion to both the foil and any paint applied onto the film. The aluminum foil is described as being so flexible that it adapts itself to the shape of the surface to be masked. The aluminum foil is described as being able to retain the deformed shape once applied to the surface that it eliminates or minimizes the necessity of using additional means for holding the mask in position. The resins that can be employed for making the mask include polyvinyl chloride, polyester and polyamide resins. Even though this masking tape is described as flexible and conformable to irregularities of a surface to be painted, it appears that the flexibility is in the direction perpendicular to the planer surface only.
Although many patents disclose masking tapes, none has been identified to describe a highly flexible, highly plasticized masking tape that is stretchable, compressible, and flexible not only in the direction perpendicular to the planer surface of the tape but also in the direction parallel to the planar surface of the tape. When a masking tape is applied to a surface in a planar, curved configuration, one edge of the tape is stretched and the other edge of the tape is compressed. If the tape is not sufficiently compressible and stretchable, the compressed edge tends to form folds and the stretched edge might have the tendency to return to its original shape by creeping on or lifting from the surface of the masked object to decrease the acuteness of the curvature. There is a need for a masking tape that is highly flexible, compressible and stretchable in a direction that is parallel to the planar surface of the tape.